Response of Lettuce Cultivars Carrying the Resistance Gene Dm11 to Isolates of Bremia lactucae from Lactuca serriola

Seven Bremia lactucae isolates from Lactuca serriola were tested for pathogenicity on 41 Lactuca sativa cultivars carrying a race specific resistance gene Dm11 which originated from L. serriola. Some of these cultivars also carried other Dm genes. Sporulation was observed in 34 % of the 287 cultivar/isolate combinations, but it was limited in most cases. A compatible response was recorded in 7.3% of cultivar/isolate. Cultivars with Dm 11 alone, or in combination with Dm 2, exhibited a higher decree of susceptibility then those also carrying Dm 3. There were differences in specific virulence among the isolates under study. Nearly all the isolates studied were virulent on newly described resistance genes Dm 15, Dm 16, and Dm 7 +? which originate from L. serriola.